Photographic cameras often contain an autofocus mechanism that can determine the optimum focus for a specified object or scene. Optimum focus is the lens position that has the smallest circle of confusion on the image plane for a given object distance. Focusing too far or to near results in a larger circle of confusion. FIG. 1 depicts focusing relative to an image plane. As depicted in FIG. 1, if a lens is focused too near an object (the bottom example) or too far from an object (the top example), the circle of confusion is larger. When a lens is focused correctly (the middle example), the circle of confusion is minimized.
However, there is not a single focus metric that can be used inside a camera to measure distance to a subject. Instead there are a variety of focus metrics used by cameras that rely on different hardware and software configurations. Some cameras use an active autofocus system to measure the distance to a subject. More commonly, cameras use a passive autofocus system to measure distance to a subject.
Passive autofocus systems determine correct focus by performing passive analysis of the image that is entering the optical system. They generally do not direct any energy, such as ultrasonic sound or infrared light waves, toward the subject. An autofocus assist beam, such as infrared light, can be used when there is not enough light to take passive measurements. Passive autofocus systems typically use phase- or contrast-based measurements to determine distance to a subject.
Generally, passive autofocus systems rely on an iterative search; in each iteration, the focus motor is stepped and the focus metric is calculated. Stepping and focus metric calculation is continued until a maximum value for the focus metric is achieved. However, to determine the maximum value for the focus metric, the search must proceed beyond the maximum until the value has begun to decline. The focus motor is then driven back to the maximum value; the system is then at its best focus. FIG. 2 depicts stepping through various focus metric measurements according to the prior art. As shown in FIG. 2, over time, the focus motor position passes maximum focus and is then returned back to maximum focus. Such a technique may take additional time, which can make it more difficult for a photographer to capture quick-moving subjects. In addition, the back-and-forth movement of physical parts can decrease battery life and can introduce unwanted wear-and-tear on components of the camera.